patch 8.2.1812: Vim9: nested closure throws an internal error

Problem:    Vim9: nested closure throws an internal error.
Solution:   Do not skip a local variable with a partial. (closes #7065)

src/testdir/test_vim9_func.vim

@@ -1388,6 +1388,20 @@ def Test_double_closure_fails()
   CheckScriptSuccess(lines)
 enddef
 
+def Test_nested_closure_used()
+  var lines =<< trim END
+      vim9script
+      def Func()
+        var x = 'hello'
+        var Closure = {-> x}
+        g:Myclosure = {-> Closure()}
+      enddef
+      Func()
+      assert_equal('hello', g:Myclosure())
+  END
+  CheckScriptSuccess(lines)
+enddef
+
 def Test_nested_closure_fails()
   var lines =<< trim END
     vim9script

src/version.c

@@ -750,6 +750,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    1812,
 /**/
     1811,
 /**/

src/vim9execute.c

@@ -377,10 +377,11 @@ handle_closure_in_use(ectx_T *ectx, int free_arguments)
 	    tv = STACK_TV(ectx->ec_frame_idx + STACK_FRAME_SIZE + idx);
 
 	    // Do not copy a partial created for a local function.
-	    // TODO: this won't work if the closure actually uses it.  But when
+	    // TODO: This won't work if the closure actually uses it.  But when
 	    // keeping it it gets complicated: it will create a reference cycle
 	    // inside the partial, thus needs special handling for garbage
 	    // collection.
+	    // For now, decide on the reference count.
 	    if (tv->v_type == VAR_PARTIAL && tv->vval.v_partial != NULL)
 	    {
 		int i;
@@ -389,7 +390,8 @@ handle_closure_in_use(ectx_T *ectx, int free_arguments)
 		{
 		    partial_T *pt = ((partial_T **)gap->ga_data)[gap->ga_len
 							  - closure_count + i];
-		    if (tv->vval.v_partial == pt)
+
+		    if (tv->vval.v_partial == pt && pt->pt_refcount < 2)
 			break;
 		}
 		if (i < closure_count)